1. Field of the Invention
The present invention relates to mobile communications, and more particularly, to a method of transmitting an overhead for a reverse packet data service in a CDMA system and to an apparatus using the same, in which, for each reverse traffic packet frame, an auxiliary pilot signal and a reverse rate indicator are alternately transmitted by a time division of one Walsh code.
2. Discussion of the Related Art
Though developed later than the TDMA system, the CDMA system has been more widely adopted and now prevails in terms of service area, but a rapidly increasing demand for packet data services has called for an update to the system standard, which was initially intended for voice and other serial data applications. The evolution of the CDMA system is ongoing and includes the standardization of the forward link, which was completed in 2002, with the standardization of the reverse link continuing into 2003. The new forward-link standard enabled high-speed packet data services through the introduction of new techniques, such as dividing a baseband frame into subpackets and applying a hybrid automatic repeat request (HARQ) transmission scheme. These techniques are similarly applicable in the reverse link, that is, in the mobile terminal.
In one example of reverse-link standardization—namely, a first evolution, data only (1xEV-DO) system—the mobile terminal determines the reverse data rate for each packet, which is divided into subpackets to be transmitted via multiple transmissions using the HARQ transmission scheme, and handles the data rate information using a control channel. In the reverse link of the 1xEV-DO system, a mobile terminal refers to a reverse activity bit (RAB) received from a base station to determine the data rate of the next baseband frame and, together with a reverse pilot signal, transmits a reverse rate indicator (RRI) for each baseband frame, i.e., each packet of reverse traffic data. The base station uses the reverse pilot signal in the detection of the data rate information, i.e., the RRI, which enables recognition of the data rate of the corresponding packet, so that the base station may perform decoding. Successful decoding requires a minimum pilot signal strength, which varies according to the data rate of the packet data corresponding to the pilot signal transmission, increasing for higher rates and decreasing for lower rates. The transmitted pilot signal, however, should allow for the maximum reverse data rate under 1xEV-DO specifications (153.6 kbps) so that a closed loop power control of the reverse link may proceed normally. That is, the same pilot signal transmission occurs regardless of the data rate—high or low—of the corresponding is packet transmission, to permit decoding at all transmittable data rates. The result is at least some degree of waste of pilot signal transmission power for any reverse-link packet data rate lower than the 153.6 kbps maximum.
FIG. 1 illustrates an exemplary reverse-link transmission in a 1xEV-DO system according to a related art, showing a pilot signal having a constant signal strength transmitted together with traffic data having a data rate varying for each frame. Thus, the transmitted pilot signal has a signal strength enabling the decoding of a 153.6 kbps traffic data packet and disregarding the potential for packet data transmission at a lower data rate, such that there is a waste of reverse pilot power when a traffic data packet is transmitted at any data rate lower than the maximum. Here, the varying data rate of the traffic data packets is represented as a variation in vertical dimension of the respective frames as depicted, to indicate more data transfer when transmitting at higher rates.
In the 1xEV-DO system adopting the method of FIG. 1, a reverse link baseband can be implemented while simplifying the associated power control and supporting the maximum data rate. Any waste of the pilot signal transmission power could be considered to have little consequence, even if the traffic data rate is much lower than 153.6 kbps. The potential for waste, however, greatly increases when applying a hybrid automatic repeat request (HARQ) transmission scheme, which permits reverse-link data rates of up to 1 Mbps. In this case, the waste can no longer be ignored since the strength of the pilot signal should be high enough to enable the decoding of the traffic packet at the highest data rate.
FIG. 2 illustrates an exemplary reverse-link transmission in which a pilot signal having an optimally set signal strength is transmitted together with traffic data having a varying data rate, showing an ideal case where a separate pilot signal strength is set for each change in data rate. The signal strength of such a pilot signal, however, would need to be variable for any subframe of reverse traffic data, which is unworkable since detection of the current reverse data rate is required before performing any power control in a closed loop power control block. To solve this, the signal of the pilot channel may be divided for simultaneous transmission via an auxiliary pilot channel and a main pilot channel, as shown in FIG. 3, whereby high-data-rate packet decoding can be achieved using the auxiliary pilot and low-data-rate packet decoding can be achieved using the main pilot, which supports closed loop power control. Such division of the pilot channel, however, necessitates allocation of a second Walsh code for use in the auxiliary pilot channel, and the resulting level of complexity represents an unreasonable exchange for saving small amounts of transmit power.